Most people don't do USB mods on the WRT54x series. SD/MMC seems way more common. Also, you'll have better luck at dd-wrt's forum for this kind of stuff, more active users there, many of which like hardware mods. Your router is running the BCM5352 chipset, as in the picture below.

I have this mod and it's quite stable. The GL has two 1.1 usb ports, all I had to do was to replace four smt resistors and solder four wires (I use external power supply). Maximum transfer speed is aprox 750KB. If you want pictures or have any questions just let me know.